The subject matter of this application is related to the subject matter of Application No. GB 9929655.0, filed Dec. 15, 1999, priority to which is claimed under 35 U.S.C. xc2xa7119 and which is incorporated herein by reference.
1. Field of the Invention
This invention relates to sensorless rotor position monitoring in reluctance machines, particularly in switched reluctance machines.
2. Description of Related Art
The control and operation of switched reluctance machines generally are described in the paper xe2x80x9cThe Characteristics, Design and Applications of Switched Reluctance Motors and Drivesxe2x80x9d by J M Stephenson and R J Blake delivered at the PCIM""93 Conference and Exhibition held in Nurnberg, Germany, Jun. 21-24, 1993 and incorporated herein by reference. In that paper the xe2x80x9cchoppingxe2x80x9d and xe2x80x9csingle-pulsexe2x80x9d modes of energization of switched reluctance machines are described for operation of the machine at low and high speeds respectively.
A typical art drive is shown schematically in FIG. 1 driving a load 19. This includes a DC power supply 11 that can be either a battery or rectified and filtered AC mains. Connected in parallel with the power supply 11 is a capacitor 25. The DC voltage provided by the power supply 11 is supplied across lines 26 and 27 and switched across phase windings 16 of the motor 12 by a power converter 13 under the control of the electronic control unit 14. For proper operation of the drive, the switching must be correctly synchronized to the angle of rotation of the rotor. One of the many known converter topologies is shown in FIG. 2, where a resistor 28 is connected in series with an upper switch 21, and with a lower switch 22 to provide a current feedback signal.
The performance of a switched reluctance machine depends, in part, on the accurate timing of phase energization with respect to rotor position. Detection of rotor position is conventionally achieved by using a transducer 15, shown schematically in FIG. 1, such as a rotating toothed disk mounted on the machine rotor, which cooperates with an optical or magnetic sensor mounted on the stator. A pulse train indicative of rotor position relative to the stator is generated and supplied to control circuitry, allowing accurate phase energization.
This system is simple and works well in many applications. However, the rotor position transducer increases the overall cost of assembly, adds extra electrical connections to the machine and is, therefore, a potential source of unreliability. In addition, at high speeds, the windage associated with the vane is a source of additional loss.
Various methods for dispensing with the rotor position transducer have been proposed. Several of these are reviewed in xe2x80x9cSensorless Methods for Determining the Rotor Position of Switched Reluctance Motorsxe2x80x9d by W F Ray and I H Al-Bahadly, published in the Proceedings of The European Power Electronics Conference, Brighton, UK, Sep. 13-16, 1993, Vol. 6, pp 7-13, which is incorporated herein by reference.
Many of the methods proposed for rotor position estimation use the measurement of phase flux-linkage (i.e. the integral of applied voltage with respect to time) and current in one or more phases. Position is calculated using knowledge of the variation in inductance of the machine as a function of angle and current. This characteristic can be stored as a flux-linkage/angle/current table and is depicted graphically in FIG. 3. The storage of this data involves the use of a large memory array and/or additional system overheads for interpolation of data between stored points.
Some methods make use of this data at low speeds where xe2x80x9cchoppingxe2x80x9d current control is the dominant control strategy for varying the developed torque. Chopping control is illustrated graphically in FIG. 4(a) in which the current and inductance waveforms are shown over a phase inductance period. (Note that the variation of inductance is depicted in idealized form.) These methods usually employ diagnostic energization pulses in non torque-productive phases (i.e. those phases which are not energized directly from the power supply at a particular moment). A method suited to low-speed operation is that proposed by N M Mvungi and J M Stephenson in xe2x80x9cAccurate Sensorless Rotor Position Detection in an S R Motorxe2x80x9d, published in Proceedings of the European Power Electronics Conference, Firenze, Italy, 1991, Vol.1, pp 390-393, incorporated herein by reference.
Other methods operate in the xe2x80x9csingle-pulsexe2x80x9d mode of energization at higher speeds. This mode is illustrated in FIG. 4(b) for motoring, in which the current and inductance waveforms are shown over a phase inductance period. It will be realized that the current waveforms for generating are mirror images of the motoring waveforms. These methods monitor the operating voltages and currents of an active phase without interfering with normal operation. A typical higher speed method is described in International Patent Application WO 91/02401, which is incorporated herein by reference.
Both the chopping and single-pulse modes described above are normally used when the converter applies a fixed value of supply voltage to the phase windings. A further mode of control is the pulse width modulated (PWM) mode, where one or more switches are switched rapidly to effectively produce a winding voltage that is proportional to the duty cycle of the PWM waveform. This allows the use of single-pulse type current waveforms at much lower speeds than would be possible on the full supply voltage. The current waveform could appear, at first sight, to be the same as that in FIG. 4(b), but closer examination would reveal that it was made up of a large number of segments, corresponding to the current carried by the switches and diodes respectively. Such operation is well known in the art and will not be described in further detail.
Having to store a two-dimensional array of machine data in order to operate without a position sensor is an obvious disadvantage. Alternative methods have been proposed, which avoid the need for the majority of angularly referenced information and instead store data at one angle only. One such method is described in European Patent Application No. EP-A-0573198 (Ray), incorporated herein by reference. This method aims to sense the phase flux-linkage and current at a predefined angle by adjusting the diagnostic point via the calculated deviation away from the desired point. Two one-dimensional tables are stored in the preferred embodiment, one of flux-linkage versus current at a referenced rotor angle and another of the differential of flux-linkage with respect to rotor angle versus current. By monitoring phase voltage and current, the deviation away from a predicted angle can be assessed, with the aid of the look-up tables, and system operation can be adjusted accordingly. However, such methods, although reducing the amount of information which has to be stored, still have to detect or compute the flux-linkage at a specific rotor angle and may be sensitive to repeatability or manufacturing tolerances in the machine.
A similar approach is disclosed in U.S. Pat. No. 5,793,179 (Watkins), incorporated herein by reference, where the arrival of the rotor at the peak of the inductance profile is predicted and the system is then put into a freewheeling mode, during which the gradient of the current is measured. A measurement of zero gradient is taken to indicate that the predicted point has been reached. While this works well in the absence of noise, it is not robust enough to disregard false readings produced by a noisy current waveform. Though the current waveform may be relatively immune to induced noise, a drive which uses a PWM voltage supply will generate a current waveform having a saw-tooth superimposed on a smooth variation. This form of waveform effectively has a large noise content. In any case, the method of ""179 cannot be used with a converter circuit which is not capable of freewheeling.
Other authors have attempted to overcome these deficiencies. One method is described in the paper xe2x80x9cA New Rotor Position Estimation Method for Switched Reluctance Motors using PWM Voltage Controlxe2x80x9d, by Gallegos-Lopez, G, Kjaer, P C and Miller, T J E, in Proc EPE""97, 7th European Conf on Power Electronics and Applications, Sep. 8-10 1997, Trondheim, Norway, Vol 3, pp 580-585, incorporated herein by reference. This paper discusses a method of continuously sampling the current waveform and attempting to detect the change in gradient which is produced by the start of pole overlap and the consequent sudden rise in inductance of the phase (see FIG. 4(b)). The basic method described by Gallegos-Lopez et al involves detecting the point of pole overlap for motoring (or pole separation for generating) by detecting where the rate of change of the current waveform, with respect to time, is zero. The method of detection uses discrete circuitry, including a differentiator, a comparator and a single shot multivibrator. The differentiator differentiates the current signal, so that at the point of zero di/dt the differentiator output is zero. The comparator is set up to detect this zero output from the differentiator and to flip state.
While the system does not require either stored magnetization data or an interval of freewheeling, it requires a high-grade form of current feedback and even then it has been found in practice that it cannot work reliably in the presence of noise, which generates spurious estimates of position. In addition, it cannot be used reliably with a low-cost form of current sensing, e.g. the resistor 28 in the bottom of the phase leg of FIG. 2, since the current information disappears when the switch is opened.
It is therefore clear that there is a need for a sensorless control method that can work with any power converter circuit using any control strategy. There is also a need for a method that does not require large amounts of stored data or expensive current feedback and is robust in the presence of noise on the waveforms from which it deduces position. Preferably, it does not require any stored magnetization data.
It is an object of the present invention to provide a robust and cost-effective method of monitoring rotor position without using a rotor position transducer while also reducing the amount of stored data.
A method in which the invention is embodied detects the position where, in motoring, the rotor poles begin to overlap the stator poles (which is the same point in generating where the poles begin to separate). Rather than follow known methods of detecting the zero slope in the phase current at that point, the point at which the current begins to fall, having reached its peak, is detected. Sufficient samples of current are taken to ensure that the true current behavior is being detected and then the point of pole overlap or separation is calculated. Since the sampling rate and calculation times are constant, it is then possible to predict accurately (given the machine speed) the occurrence of the pole overlap in the next phase to be energized. The sampling of current can be done with a low-cost current sensor and, if PWM operation is being used, the sampling can be synchronized with a particular switch state, thus allowing chopping with the switch adjacent to the current sense resistor, if required.